1. Field of the Invention
The present invention relates to a vehicle control system and more specifically to a control system which enables the engine to be electronically controlled in response to an accelerator position.
2. Description of the Prior Art
Japanese Patent Pre-publication No. 60-43133 discloses a system wherein the engine is electronically controlled in response to the vehicle accelerator. In this system the driven wheel speed and the non-driven wheel speeds are sensed and used to detect the vehicle tire slip ratio. The derived ratio is compared with a predetermined value and, in the case the instant slip value exceeds the predetermined one, the amount of fuel supplied to the engine is controlled in accordance with the accelerator position.
With this arrangement, in the event that the condition of the road on which the vehicle is running changes, for example from wet to snowy conditions, and the detected slip ratio exceeds the predetermined value, the amount of fuel supplied to the engine is reduced to lower the engine output power and thus control the situation. However, if the road conditions then change back to ones wherein good traction occurs and the slip ratio diminishes, the amount of fuel supplied to the engine is increased in a manner to re-establish the engine output to the level provided before the poor road conditions were encountered irrespective of the accelerator manipulation. However, this control induces the drawback that, even though the accelerator depression remains constant, the amount of fuel supplied to the engine varies in a manner which causes the engine to surge and produce more torque than expected and induces a situation wherein the driver experiences a disconcerting loss of correlation between the accelerator pedal position and the operation of the engine.
In addition to the above, if wheel slip is encountered and the control system reduces the amount of fuel supplied to the engine and corrects the problem, the control system subsequently effects steps to increase the engine output. However, even though the system is controlled in accordance with the accelerator position parameter, even if the driver subsequently deeply depresses the accelerator to obtain vehicle acceleration, the system (during this correction period) fails to respond quickly and the engine output increases gently and below the rate that would be expected for the given depression. This again gives rise to the problem that the desired response to accelerator depression is not derived and the driver experiences a disconcerting loss of control over the engine.
With the above system, if the engine is operating with the accelerator pedal depressed by only a small amount, and the traction between the vehicle tires and road lowers (viz., due wet or icy conditions) the amount of fuel supplied to the engine is decreased. However, when traction subsequently improves the amount of fuel supplied to the engine is increased in a manner which produces a slightly excessive amount of torque and induces wheel slip even though the depression of the accelerator remains unchanged. This tends to increase the instant slip ratio above the predetermined level and induces a reduction in the amount of fuel supplied to the engine. This phenomenon tends to cyclically repeat itself inducing the engine torque to hunt and cause the vehicle to lurch and/or shudder unpleasantly.